After a well has been drilled there are often situations where the well must be sealed off. This is normally preformed with a cement plug. Other situations that may require sealing off an area of a well include: when a side track is needed during the drilling process; curing lost circulation during drilling; when a zone must be permanently isolated; and when a wellbore is going to be plugged and abandoned.
The cement plug is typically constructed by pumping a small amount of cement slurry down a drill pipe where it later sets solid to form a plug. Drilling operations can resume when the cement plug has set and developed enough strength to fulfil its objective.
In most situations the cement plug must be located some distance off the bottom of the wellbore in either a tubular or open hole section of the well. In order to achieve this, a viscous pill, generally composed of a dense viscous bentonite and sodium silicate mixture, is spotted in the borehole, to support the cement plug. A variation on this method includes the use of an “umbrella” device placed under the drill pipe to support the plug instead of the pill. Alternatively an expanding basket with a membrane over it can be inserted into the wellbore in front of a dart that can be pumped down a drill pipe. The basket expands below the drill pipe on top of which cement is placed.
Problems with these methods are that they result in two heavy fluids stacked on top of a light fluid. One or both fluid interfaces can become unstable leading to inversion. The heavy fluid(s) move down one side of the borehole and the light fluid fluids move up the other side. Considerable mixing and contamination may then occur during the movement of the fluid. The channel formed prevents a pressure seal from being achieved and also significantly reduces the plug's mechanical strength. The mixing of the fluids can also destroy the mechanical strength of the plug. Cement plugs can fail up to third of the time. In lost circulation situations failure rates of cement plugs can be as high as 50%.
Determining whether the plug setting is successful at all, i.e. if the cement is hard enough to side track against and/or if the cement is hard enough to pressure test against, adds significant rig time due to the waiting on cement. Also cement plugs are only placed through drill bits in certain well control situations, and all other situations require tripping out. Thereby requiring installation of the production tube at the bottom of the string, running in and placing the plug. If the cost of tripping out is very high the string may be left in place during the waiting period and then used to tag the plug, in this case a new plug may be placed immediately. Otherwise the driller will trip out and trip back in the drill bit. Once the waiting time is over the driller will try to tag the cement. If the cement is still soft, the driller will wait for longer. As there is great uncertainty about when it is safe to resume drilling operations, to mitigate the risk of resuming drilling operations too soon, it is common practice to wait extra time, typically 24 hours, before resuming drilling.
Therefore it is an object of the invention to provide a method for monitoring a cement plug during in real time to determine the properties of the cement plug.